Approximately 2% of the population suffer from psoriasis, a flaking of the skin caused by a gene defect. A greatly increased rate of cell division (cell proliferation) in the basal layer of the epidermis gives rise to the flaky focus of the psoriasis which leads to severe physical and psychological effects.
The object of psoriasis therapy is to cause the focus to recede in order to make the skin free of flakes and keep it in this condition. At present therapy takes the form of treatment by chemotherapy, photochemotherapy and phototherapy.
In chemotherapy medication is orally administered or applied to the skin. In most cases this involves not only considerable side effects but also pain. There is also the disadvantage that chemotherapy can only be carried out on an in-patient basis in clinics.
Photochemotherapy uses medication in combination with UV radiation. In the Goeckermann therapy which has been used for a long time, tar is applied to the skin and then UV irradiation is carried out. In order for therapy to be successful, radiation must be applied until the skin shows an erythema (sunburn).
A modern type of photochemotherapy is known as PUVA. In this form of therapy a photosensitising medicament (Psoralen) is orally administered or applied to the skin. This is followed by irradiation with UV-A in the wave length range between 310 and 440 nm. The photosensitising medicament serves to make the skin more sensitive to the longwave UV-A radiation.
In this known method devices used as UV radiation sources either contain UV emitting fluorescent lamps (UV-A low pressure radiation devices) or mercury high pressure radiation devices, xenon radiation devices or mercury high pressure radiation devices doped with metal halides. The shortwave radiation below 315 nm is generally suppressed by filters in these devices.
In order for therapy to be successful, in this known method an erythema must also be produced, and this causes pain. In addition the skin--and, in the case of oral administration of Psoralen, also the eyes--cannot be exposed to daylight for several hours after administration of the medicament since otherwise considerable damage can occur. This means that the patients must either stay for several hours in the clinic or must be treated as in-patients. Depending upon the skin sensitivity the radiation times range from a few minutes to of the order of one hour.
On average this PUVA therapy requires 20 to 25 irradiation sessions, and in the course of the therapy a considerable browning of the skin (pigmentation) takes place. In the case of oral administration of Psoralen it is also of significance that a not inconsiderable number of patients show a poor toleration of this medicament.
In contrast to this photochemotherapeutic method, in phototherapy the therapeutic effects of UV radiation are used without the additional use of medication.
Earlier methods used mercury vapour high pressure radiation devices (home sunlight, high sunlight). Nowadays there are a number of different UV radiation sources with different spectral distribution. For psoriasis therapy UV fluorescent lamps are used which are known as UV-B lamps ("Sunlamp") and their emission spectrum in the UV range is from approximately 270 nm to approximately 365 nm. More than half of the UV energy is radiated in the range between 270 and 315 nm.
Some UV fluorescent lamps are also used in which the UV-B proportion (under 320 nm) is a very low percentage whilst the UV-A proportion (over 320 nm) is very high. These lamps are frequently known as UV-A lamps ("Blacklight"). Their emission spectrum in the UV range is from approximately 300 to 400 nm.
For the so-called SUP therapy (selective UV therapy) mercury vapour high pressure radiation devices doped with metal halide vapour are mostly used in which the emission spectrum in the UV range extends from approximately 250 nm to 400 nm. The wavelength range from approximately 290 to 335 nm is regarded as therapeutically effective.
In this phototherapeutic method radiation times which do not cause any erythema (sunburn) are used at first. However, in the course of the phototherapy the dose is increased so that erythema occurs. The minimal erythema is the slightest perceptible reddening of the skin; it is also known as the erythema threshold. The more intense an erythema is the more unpleasant are its side effects. These correspond to the side effects of sunburn or of burning (tightening, itching, pain and reddening, peeling, blistering, fever).
A further method of therapeutic treatment of psoriasis is known as SHIP (super high-intensive phototherapy). It makes use of the wavelength range from 320 to 330 nm. The proportion of the radiation lying below 320 nm is largely filtered out, but not the radiation above 330 nm. The radiation times are in the region of half an hour. On average approximately 30 radiation sessions are necessary before therapy is successful. In this method also a pigmentation of the skin occurs as a side effect.